Abstract

The southern Sierra Nevada foothills, central California (USA), expose a fossil pre–40 Ma bedrock pediment which we call the southern Sierra Nevada pediment. We document this landscape with multiple types of data, and also report new apatite ^4He/^3He, (U-Th)/He, and zircon (U-Th)/He data from the pediment that significantly expand the spatial extent of southern Sierra low-temperature thermochronology data westward into the foothills. Applying recently published thermal modeling software for thermochronologic data, which uses a transdimensional Bayesian Monte Carlo Markov chain statistical approach, we tightly constrain the thermal history of the southern Sierra Nevada pediment. Integrating this thermal history with numerous previously published data sets from across the southern Sierra, we present a chronology of tectonic and landscape evolution of the southern Sierra Nevada. For the first time we cover the entire width of the range, integrate the numerous published data sets into a single coherent geologic story, and link each phase of this story to a potential mechanism.
Modeling results are consistent with a three-phase cooling history for the southern Sierra Nevada pediment. Rapid exhumation ca. 95–85 Ma resulted in cooling to between 55 °C and 100 °C. Following this, slow cooling to surface conditions occurred from 85 Ma to 40 Ma at rates consistent with those estimated for the axial southern Sierra during the same time period by previous studies. Little if any additional cooling occurred post–40 Ma. We hypothesize that a thin sedimentary cover protected the 40 Ma bedrock landscape through much of the last 40 m.y., and that this cover eroded away post–10 Ma, re-exhuming the southern Sierra Nevada pediment as a fossil pre–40 Ma landscape. Each of these three phases of cooling links to a distinct tectonic or geomorphic regime, including the profound rapid exhumation of the southern Sierra Nevada–Mojave segment of the Cretaceous arc due to subduction of a large oceanic plateau, the formation of the low-relief landscape of the high-elevation areas of the southern Sierra Nevada with more limited tectonic forcing, and Eocene activity on the Western Sierra Fault System.